Microwave is the electromagnetic wave of which wavelength is between the wavelength of infrared light wave and the wavelength of radio wave, i.e.: within the range of 1 mm-100 cm.
In a microwave field, the molecules of the medium generate dipole orientation polarization and interfacial polarization. As the alternating electric field generated by microwave changes direction hundreds of million times per second and dipole orientation polarization is unable to keep pace with the fast changing alternating electric field and lags behind it, intermolecular friction and heat energy are generated. The energy is transferred in form of electromagnetic wave to realize agitation at molecular level and fast and uniform heating, so microwave heating is also known as “voluminal heating” without a temperature gradient.
Compared with conventional heating methods, microwave heating has the following advantages: 1. Voluminal heating. Under the action of microwave, heat may be generated in different depths of the acted medium simultaneously, so that the heating of the medium is uniform without a temperature gradient so as to realize steady chemical reactions of the heated medium. 2. Selective heating of medium. For metal (conductor) medium, microwave may be completely reflected, so it is not easy to heat; for the medium with low electric conductivity and small polarization loss, microwave is almost completely transmitted through it, so it can not be heated; for the medium which is prone to polarization in a microwave field, it can easily absorb microwave and thus can be quickly heated. 3. High thermal efficiency and no pollution. Microwave can convert electromagnetic energy into heat energy without generating pollution or energy loss.
In addition, microwave also has a special “non-heating effect” resulting from its direct action upon the reacting molecules. As proved by tests, microwave has such special effects as: changing the course of chemical reactions, lowering reaction activation energy, increasing synthesis velocity, raising equilibrium conversion, reducing byproducts and changing stereoselectivity of the products. Just because of the special promotion effect of microwave on chemical reactions, the application of microwave in chemical reactions not only has the great significance of theoretical study but also has huge potential in industrial application.
Due to the above features of microwave, microwave has been extensively researched and applied as an efficient and clean heating means and chemical reaction means in recent years. However, since the penetration distance of microwave is short and the temperature of the heated medium is uncontrollable under the continuous irradiation of microwave, large-scale industrialization can't be realized by using the microwave devices currently applied in chemical reactions and microwave heating for chemical reactions only stay in the stage of laboratory study.
CN2821468Y discloses a microwave treater at least comprising a box-shape cavity which is provided with conduit joints in the middle locations of its two opposite sides. Among the other two opposite sides of the box-shape cavity, one is closed and the other side is a flange joint. This flange joint is connected to a microwave generator. The conduit through which the fluid to be heated passes is disposed inside the box-shape cavity, and the two ends of the conduit extend out from the conduit joints. A plurality of such treaters (≦15) may be connected to form a long reactor. Meanwhile, a plurality of conduits may be arranged in parallel inside the cavity. This device is designed considering the problem of short penetration distance of microwave, but it still fails to solve the problem of uncontrollable medium temperature under the continuous irradiation of microwave, so it can be used to heat fluid only, but can't be applied in the chemical reactions in which temperature needs to be controlled under the continuous irradiation of microwave.
CN1231213A discloses an industrial microwave oven exclusively for fluid treatment which comprises a microwave resonant cavity provided with a microwave input port, a fluid inlet, a fluid outlet and an operation gate, and the shields disposed at the fluid inlet and outlet, respectively. Inside the resonant cavity, a fluid circulator exclusively for making the fluid subject to full physical and chemical reactions in the microwave field is disposed. In the upper part and lower part of the resonant cavity, the devices enabling the continuous supply and discharge of fluid as per process requirements are connected to the fluid inlet and outlet. Although the microwave oven guarantees the continuous supply and continuous discharge of material, it can heat the fluid only and can't be applied in the chemical reactions in which temperature needs to be controlled under continuous irradiation of microwave.
The design in the above patent literatures realizes the industrialized application of microwave as a heating means, and solves the problem of poor penetrating ability of microwave, but these devices still fail to solve the problem that the temperature of medium increases continuously and rapidly and can't be controlled under the continuous irradiation of microwave, so they can't be applied in the chemical reaction processes in which temperature needs to be controlled under the continuous irradiation of microwave.